1. Field of the Invention
This invention relates to a process of etching plastics for painting and plating. The etched plastics can be plated with conventional plating processes. The etched plastics can also be painted with water based paints.
2. Brief Description of Prior Art
Plastics have been metallized/plated for decorative and functional (e.g., EMI shielding) purposes. Several plastics, such as copolymers of acrylonitrile, butadiene and styrene (referred herein as to ABS), alloys of ABS (e.g., an alloy of ABS and polycarbonate), polycarbonate, polyvinyls, such as polyvinylchloride, polyolefins, such as polybutadiene, polyepoxide, polyphenylene sulfide, and polyphenylene oxide have heretofore been etched with chromic acid for metallization. Chromic acid is highly toxic, expensive, and creates sludge. There is a need for formulations and processes which are less toxic, less polluting, and less expensive for etching plastics for plating and painting.
A number of substitutes have been proposed for chromic acid for etching plastics, such as ABS, polyamides, polyesters, polyepoxy resins, polyimides and polyvinylchloride for plating. They include bases, such as NaOH (U.S. Pat. Nos. 3,554,880; 3,770,528; 3,791,848; 4,131,698; 4,422,907; 4,505,786; 4,528,245; 4,592,852; 4,629,636; 4,725,504; 4,803,097; 4,820,553; 4,832,799; and 4,959,121), NaOCl (U.S. Pat. Nos. 3,460,897; 4,592,852 and 4,629,636), hydrogen peroxide (U.S. Pat. No. 4,645,573), nitric acid (U.S. Pat. Nos. 3,471,320; 4,959,121; 5,198,096 and 5,160,600), phosphoric acid, acetic acid and chromic acid (U.S. Pat. Nos. 3,484,270 and 3,522,073), molybdic acid (U.S. Pat. Nos. 3,563,163 and 3,567,532), potassium permanganate (U.S. Pat. Nos. 3,598,630; 3,652,351; 3,819,394, 4,042,729; 4,073,740; 4,859,300; 4,959,121 and 5,112,513), and sulfur trioxide (U.S. Pat. Nos. 4,298,636; 4,520,046 and 4,556,587).
Prior to etching plastics for electroless plating, they are sometimes pre-treated with a solvent system to swell the surfaces. Solvent pretreated/swollen surfaces are micro-toughened and hence are relatively easy to etch and hence provide better adhesion of the plated metals. A number of solvents, such as butyrolactone, propylene carbonate, glymes, and methanol, have been proposed in a number of patents (e.g., U.S. Pat. Nos. 3,445,350; 3,689,303; 3,769,061; 3,795,622; 3,963,590; 4,063,004; 4,131,698; 4,775,449; 5,019,425; 5,132,191; 5,178,956; 5,180,639; and 5,185,185). However, in order to get improved adhesion, the solvent must be removed from the surface and the surface still requires etching. There is a need for an etching formulation which simultaneously swells the surface.
Nitrosyl sulfuric acid, [HO(O.dbd.)S(.dbd.O)ONO or NO.sup.+ HSO.sup.4 --], which is also known as nitrosyl sulfate, chamber crystals, nitrososulfuric acid, nitroxysulfuric acid, nitrosulfonic acid, nitrosyl hydrogen sulfate, nitro acid sulfite, and nitrose is mainly used for preparing azo derivatives of aromatic amines (e.g. U.S. Pat. No. 5,162,511). Nitrosyl sulfuric acid is referred to herein as NSA. Solid NSA decomposes at 73.degree. C. NSA decomposes with formation of HNO.sub.3 and H.sub.2 SO.sub.4 upon exposure to moisture. It is soluble in sulfuric acid and decomposes violently upon addition of water.
NSA is known to react as a nitrosyl cation or nitrosonium cation (NO.sup.+) with other nucleophiles including unsaturated compounds, such as 1,3-dienes [Chem. Ber. 99, 556 (1966)]. NO.sup.+ is also capable of halogen abstraction from alkyl halides [G. A. Olah et al, Synthesis, 274 (1979) and 713 (1983)]. NO.sup.+ is an mild oxidizing agent. It is a powerful one electron oxidant [Lee et al, Inorg. Chem., 29, 4196 (1990) and Kim et al, J. Amer. Chem. Soc., 113, 4962 (1991)] and is therefore used as an oxidative dopant for polymers. It has been used to oxidatively cleave oximes, hydrazones, thioketals, ethers etc. to their corresponding carbonyl compounds [Synthesis, 418 (1977), 273 (1979), 609 and 610 (1976), J. Org. Chem., 42, 309 (1977)]. The NO.sup.+ is electrophilic, and therefore reacts with a large number of basic molecules. The nitrosonium cation also forms adducts with crown ethers. Though the reactions of NSA have been reported with a number of small molecules, there is no report on its reactions with polymers, especially for etching and chemical modification of the polymers.